I think the date airlines stop allowing you to fly without a note from your dr is 36 weeks, not 26. Same goes for my dr's advice-not in the last 4 weeks of pregnancy, but even that he's flexible on depending on the case. When I was pregnant with my daughter we flew to Florida and I was 30 weeks but looked about 45 weeks and not one person even asked me a question. This pregnancy we'll be flying to my BIL's graduation when I'm 26-27 weeks and I know American's policy is 36 weeks so we're fine there. Cruise ships though stop at 26 or 18 weeks I believe.
That said, I went to WDW at 19-20 weeks with my daughter and 16-17 weeks this time, and I would go again later in my pregnancy if it weren't for the fact that it will be the summer and way, way too hot for me. If it were January though I'd go without hesitation during my 3rd trimester.
As far as taking a newborn, we took my daughter for her first trip May 19, 2004 and she was born February 26, 2004, so she was not quite 3 months old. She was due March 12th though so shoudl could have been even younger cause the dates were set before she was born. It was the easiest trip I've ever taken with her and I have no regrets. Our pediatric group says sunblock is fine on babies under 6 months old so we used it everyday with Madison without a problem, but kept her completely shaded at all times anyway so we didn't really need the sunblock. With this baby, we are planning our 1st trip for early December and he's due August 28th (with the expectation that he'll be induced around 37-38 weeks so more of a mid-August birthday) so he should be around 3-3.5 months old. But this
free dining thing is really making me want to plan a trip earlier, around 9/30-10/4, and if we do end up doing that he'll be less than 2 months old. Now, with my daughter that was an easy trip, but with a 2.5yo and a newborn I'm sure it will be different. We're still thinking about it, but clearly since we're even entertaining that idea we have no objections to taking our newborn. And my husband is a dr (well, he will be when he graduates on 5/21) and therefore biggest germophobe you could ever meet, but he doesn't have a problem with taking our newborn son along either.
So, after my long-winded opinion, bottom line, yes, I'd go. You'll have family to help you care for the newborn if he/she is already born, or if you're pregnant you'll have some help with your other 2 kids. It might not be the easiest trip in the world, but definitely won't be the hardest either and I think the benefits outweigh any negatives.